1. Field of the Invention
The present invention relates in general to vehicle anti-lock brake systems, and particularly to a speed sensor ring used in such systems.
2. Description of Prior Developments
Anti-lock braking systems commonly include a vehicle speed sensor assembly for generating a pulsed electrical signal related to vehicle speed. During a vehicle braking operation, the speed signal is sent to an on-board computer that responds to the drop in vehicle speed. If there is an impending wheel lock-up condition, the computer directs a valve in the brake system to relieve or modulate the fluid pressure within the rear wheel brake cylinders thereby preventing the undesired wheel lock-up condition. When the computer determines that a lock-up condition is no longer imminent, the braking pressure is returned to normal.
The speed sensor assembly commonly includes an annular metal rotor attached to the brake drum so as to be rotatable with the road wheel. The metal rotor has a large number of radial slots evenly spaced around its rotor circumference. Typically, there may be about one hundred such slots.
The speed sensor assembly further includes a stationary voltage generator that generates an A.C. signal as the rotor passes across it. The voltage generator or speed sensor contains a permanent magnet and an electrical coil such that the radial slots of the rotor disrupt the magnetic field to provide an A.C. signal in the coil related to road wheel speed. The signal is sent to an on-board computer for controlling the vehicle braking operation.
The slotted metal rotor usually includes a radial or axial wall containing the slots and a circular flange or cylinder encircling the wheel hub. In one case, five or more ears may project radially from the rotor outer edge for mounting the rotor on the rear wall of the brake drum or housing.
The slotted metal rotor is somewhat costly to manufacture. Also, the installation cost of the rotor can be significant due to the fact that several screws have to be extended through the mounting ears into threaded holes in the rear wall of the brake housing or a separate crimping operation or press fit is required.